1. Field of the Invention
This invention relates to a positive temperature coefficient (hereinafter referred to as "PTC") heating device, and more particularly to a PTC heating device which is adapted to be used as a heat source for an electric vaporizer which is adapted to heat liquid material, e.g., liquid insecticide, aromatic liquid, or the like, to vaporize it. Such electric vaporizer includes an electric mosquito destroyer, an electric vaporizer for heating aromatic liquid to vaporize it, and the like. As an example of such an electric vaporizer having a PTC heating device incorporated therein, an electric mosquito destroyer will be referred to in the following.
2. Description of the Prior Art
This type of electric mosquito destroyer generally comprises a reservoir having liquid insecticide contained therein, and an wick for sucking up the liquid insecticide from the reservoir, in which electric mosquito destroyer the liquid insecticide having been sucked up from the reservoir through the wick is adapted to be heated to vaporize. In the electric mosquito destroyer, a PTC thermistor which is capable of safely heating the liquid insecticide to reliably vaporize it is employed. The wick usually has a body which is made of fibrous material such as felt, or porous material.
Referring now to FIG. 1, this type of electric mosquito destroyer having a PTC heating device incorporated therein will be described more in detail in order to facilitate understanding of the present invention. The electric mosquito destroyer comprises upper and lower armoring cases 40 and 42 which are adapted to be fitted into each other, and a reservoir 44 having liquid insecticide contained therein, the reservoir 44 being housed in the lower armoring case 42. The upper armoring case 40 has a through-hole 40a formed at its upper wall 40b, and socket portions 40c integrally formed on an inner surface of its side wall. As described above, the electric mosquito destroyer further comprises the PTC heating device 500 which includes a casing 50 and a cylindrical heat-radiating member 90 (see FIG. 2) which is mounted with respect to the casing 50 as will later be described in detail. The PTC heating device 500 is mounted with respect to the upper armoring case 40 in a manner such that the casing 50 thereof is partially fitted in the respective socket portions 40c of the upper armoring case 40. In FIG. 1, a reference numeral 46 designates a wick for sucking up liquid insecticide from the reservoir 44, which wick has an elongated body made of fibrous material such as felt, or porous material as described above. The elongated wick 46 is inserted into the reservoir 44 through a mouth of the reservoir 44 to be immersed in the liquid insecticide contained in the reservoir 44, an upper end portion of which elongated wick 46 is projected from the mouth of the reservoir 44. The upper armoring case 40 having the PTC heating device 500 mounted with respect thereto and the lower armoring case 42 are fitted into each other in a manner such that the upper end portion of the elongated wick 46 is inserted through a longitudinal bore 92 (FIGS. 1 and 2) of the cylindrical heat-radiating member 90.
In the electric mosquito destroyer constructed as described above, when heat is generated at a PTC thermistor, which is incorporate in the PTC heating device 500 as will later be described in detail, to be transmitted to the heat-radiating member 90, the upper end portion of the wick 46 having sucked up liquid insecticide from the reservoir 44 is heated due to the heat which has been conducted to the heat radiating member 90, resulting in the liquid insecticide vaporizing. Thus, the vaporized insecticide is diffused out of the electric mosquito destroyer through the through-hole 40a of the upper armoring case 40.
Referring to FIG. 2, a conventional PTC heating device 500 comprises a PTC thermistor 80, an upper electrode plate 70 and a lower electrode plate 60 between which the PTC thermistor 80 is vertically interposed, a casing 50 having a recess 51 formed therein, and a cylindrical heat-radiating member 90.
The casing 50 is made of heat-resistant plastic material and comprises a body having the recess 51 and a cylindrical portion 52 serving as heat-conducting section which is formed integrally with the body so as to protrude upwardly from a substantially central portion of an outer bottom surface of the recess 51. The PTC thermistor 80 has a ring-like body and an upper electrode 80a and a lower electrode 80b on an upper and a lower surface of its body, respectively. The PTC thermistor 80 and the electrode plates 70 and 60 are received in the recess 51 of the casing 50 in a manner to be fitted on the cylindrical portion 52 of the casing 50 through their through-hole 82, through-hole 72 and through-hole 62, respectively. In FIG. 2, a reference number 100 designates a cover plate. The cover plate 100 is made of heat-resistant plastic material, has a through-hole 101, and is arranged on the upper electrode plate 70 in a manner such that its through-hole 101 is aligned with a longitudinal bore 52a of the cylindrical portion 52 of the casing 50 through its, thereby blocking up the recess 51 of the casing 50. The cylindrical heat-radiating member 90 is made of metal such as aluminum and provided at its one end with a flange 91. The heat-radiating member 90 is mounted with respect to the casing 50 in a manner to be inserted through the through-hole of the cover plate 100 and the longitudinal bore 52a of the cylindrical portion 52 of the casing 50 to be riveted over the outer bottom surface of the casing 50 at the other end thereof.
The conventional PTC heating device constructed as described above has various problems as will be described hereinafter.
As described above, the conventional PTC heating device 500 includes the ring-shaped PTC thermistor 80. In the manufacture of the ring-shaped PTC thermistor, it has been the practice to form the electrodes by electrolessly plating the entire surface of ring-shaped substance for the PTC thermistor with, e.g., nickel and then heat-treating same. Thereafter, nickel film on an inner wall surrounding a through-hole of the ring-shaped substance is removed by, for example, grinding. The grinding operation is, however, highly troublesome, thus requiring substantial time for the removal of the nickel film. Therefore, the ring-shaped PTC thermistor has become expensive. As an alternative, removal of the nickel film from the inner wall may be carried out by chemical etching. This, however, causes the ring-shaped substance and electrodes formed on the ring-shaped substance to deteriorate, because the etching operation is carried out using chemicals.
Further, in the conventional PTC heating device, the cylindrical portion 52 of the casing 50 and the cylindrical heat-radiating member 90 inserted through the longitudinal bore 52a of the cylindrical portion 52 extend in a direction at right angles to horizontal surfaces of the electrodes 80a and 80b of the PTC thermistor 80, so that heat generated at the electrodes 80a and 80b of the PTC thermistor 80, when the PTC heating device 500 is in use, will follow several paths to be transmitted to the heat-radiating member 90 serving as means for heating liquid insecticide having been sucked up the wick 46. More particularly, in the conventional PTC heating device, the horizontal surfaces of the electrodes 80a and 80b of the PTC thermistor 80 are out of contact with the cylindrical portion 52 and are in contact with the cover plate 100 and a bottom portion of the casing 50 through the upper electrode plate 70 and the lower electrode plate 60, respectively, so that heat generated at the electrodes 80a and 80b of the PTC thermistor 80, when the PTC heating device 500 is in use, will be transmitted through the upper electrode plate 70 and the lower electrode plate 60 to the cover plate 100 and the bottom portion of the casing 50, and then conducted to the heat-radiating member 90 through the cylindrical portion 52, thereby introducing a delay in transmitting the heat to the heat-radiating member 90. Therefore, in the conventional PTC heating device, it will take long for the heat-radiating member 90 to be heated to a temperature sufficient to vaporize liquid insecticide.
Also, a cylindrical constant temperature heating device which is similar to the above-described conventional PTC heating device and adapted to be used as a heater for preventing the freezing of a water-supply pipe or the like is found in Japanese Laid-Open Patent Publication No. 35490/1985. FIGS. 3 and 4 shows the constant temperature heating device. In FIGS. 3 and 4, components which are identical to those shown in FIG. 2 are identified by the same designators and the description of them will not be repeated. A reference numeral 300 designates a lead wire which is connected to the upper electrode 80a of the PTC thermistor 80 by solder or conductive adhesives 302, and a reference numeral 400 designates a lead wire which is connected to the lower electrode 80b of the PTC thermistor 80 by solder or conductive adhesives 402. This constant temperature heating device also has the above-described problems.